This invention relates to boiler economizers.
Economizers are commonly employed in connection with boilers to recover a portion of the heat contained in the exiting flue gases. Typically, such economizers include a heat exchanger in the flue which preheats the boiler feedwater. One problem with such economizers is that if the temperature of the flue gas is lowered below the dew point of entrained acid vapors, highly corrosive acids may be formed. In order to minimize such corrosion, many prior art economizers limited the amount of heat recovered from flue gases, thereby lowering overall system efficiency.
One prior art type of economizer system which attempted to increase heat recovery during high boiler firing rates also included ductwork and dampers for routing the flue gases around the economizer at lower boiler firing rates at which corrosive acids might normally be formed. The economizer would then be sized for operation at higher firing rates only. This type of system was not wholly satisfactory, however, because heat could not be recovered from flue gases bypassed in this manner. Further, such ductwork is relatively costly to install and in addition, could not be employed universally because of substantial space requirements.
Another prior art economizer diverts a portion of the feedwater around the economizer at lower firing rates. With a smaller quantity of water flowing through the economizer, the flue gas temperature tended to remain higher. However, in many instances, the inlet water temperature would be cool enough to allow some flue gas condensation, causing cold end corrosion. Also, if the economizer size is increased for maximum heat recovery, low fire flow can be so low as to cause steaming.
Yet another prior art economizer system controls inlet water temperature by the use of an external heating source. Heat was added to the inlet water by means of steam heat or a heat exchanger located in the lower boiler drum. The effectiveness of such systems was limited by the steam or boiler water temperature, which in low pressure boilers, may not be sufficient to prevent corrosion. In addition, such systems could not be substantially self-contained because of the requirement for additional piping. Further such systems had a poor response time and were difficult to control.